Internal combustion engines produce exhaust gasses containing by-products of inefficient or incomplete combustion. Among these by-products are such pollutants as carbon monoxide, nitrous oxides, and a variety of hydrocarbons. Even diesel engines, which use a higher air-to-fuel ratio than do gasoline (Otto cycle) engines, produce excessive nitrous oxides along with hydrocarbons and carbon monoxide. These combustion by-products are undesirable because they are both harmful to the environment and wasteful.
Carbon monoxide is a known greenhouse gas and is also toxic in large quantities, since it is preferentially absorbed over oxygen in red blood cells. Some nitrous oxides are also toxic, and contribute to acid rain. And among the plethora of hydrocarbons produced by inefficient combustion are carcinogenic benzpyrene and nitroaromates. Inefficient combustion is also wasteful insofar as the carbon monoxides, nitrous oxides, and hydrocarbons may yet be further oxidized to release potential chemical energy stored within.
One known method of removing the by-products of inefficient combustion is to pass the exhaust stream through an exhaust gas recirculation (EGR) system, as illustrated schematically in FIG. 1. In general, EGR systems 10 direct a portion of the gas exiting an engine exhaust manifold 12 back into the engine intake manifold 14, such that the recirculated exhaust gas may participate in the combustion process a second time. At least some of the by-products of inefficient combustion present in the recirculated exhaust gas are completely oxidized as they once again pass through the engine 16 and are exposed to the combustion process ongoing therein. While this method is efficient in reducing the level of inefficient combustion by-products (especially nitrous oxides and carbon monoxide) ultimately emitted by the engine 16, the exhaust gasses must first be cooled before being reintroduced into the engine 16 in order to control the combustion process. Cooling is accomplished by routing the hot exhaust gasses through a cooling chamber 18. One requirement of any EGR system 10 is that the pressure of the gasses exiting the engine exhaust manifold 12 be greater than the pressure of the gasses entering the engine air intake manifold 14 so that the exhaust gasses will flow into the intake manifold 14. Compressor 20 and turbine 22 sizes and efficiencies determine this manifold pressure gradient in the absence of a pressure assist device. Without a pressure assist device to maintain the exhaust gas at a pressure greater than the intake manifold pressure, the EGR system 10 will not function under all engine conditions (such as when torque is relatively high and revolutions are relatively low--see FIG. 2). Electronic throttle systems for maintaining an elevated exhaust gas back-pressure are known in the art. One such system is described in U.S. Pat. No. 5,806,308 to Khair, et al., EXHAUST GAS RECIRCULATION SYSTEM FOR SIMULTANEOUSLY REDUCING NO.sub.X AND PARTICULATE MATTER. These systems include electromechanically actuates throttle mechanisms. The electromechanical throttle mechanism requires microprocessors for performing the control functions. These devices also require gas pressure sensors (such as a differential pressure sensor or a mass flow transducer) for measuring the exhaust gas flow rate or pressure. The exhaust gas data is transmitted to the microprocessor, which calculates the current exhaust gas pressure, compares the calculated exhaust gas pressure to the desired exhaust gas pressure, and actuates throttling to approach the desired exhaust gas pressure. One disadvantage of systems of this type is the requirement of a plethora of electrical devices, such as the electromechanical throttle, microprocessor, and sensor. In addition to adding bulk and expense, such a system is vulnerable to the degradation or failure of any one of a number of electrical connections. Further, the increased complexity of the system increases the time and expense of repairs when a problem develops.
There is therefore a need for a cheap and reliable system for controlling exhaust gas pressure such that exhaust gas pressure is substantially always maintained at a desired level. The present invention is directed towards meeting this need.